High-speed tool steels

ABSTRACT

A high-speed tool steel possessing high wear resistance combined with good ductility and machinability, consisting essentially of 0.8 to 1.8 percent carbon, 3.5 to 10 percent chromium, 1 to 13 percent tungsten, 1-10 percent molybdenum, 0.5 to 5 percent vanadium, 0 to 10 percent cobalt, balance iron, wherein the silicon content is not less than 0.5 percent and is determined by the formula: % Si 2.7 (%C) - 0.1 (%W) - 0.14 (%Mo) - 0.5 (%V) + 0.3.

United States Patent 191 Haberling et al.

1 HIGH-SPEED TOOL STEELS [75] Inventors: Edmund Haberling; HeinrichKiesheyer, both of Krefeld; Hanshermann Weigand, Tonisvorst, all ofGermany [73] Assignee: Deutsche Edelstahlwerke Gesellschaft mitbeschrankter Haftung, Krefeld, Germany 22 Filed: Dec. 26, 1973 211 Appl.No.: 428,312

[30] Foreign Application Priority Data Dec. 27, 1972 Germany 2263576[52] US. Cl. 75/126 A, 75/126 C, 75/126 E, 7 75/126 H, 75/126 .1, 75/126Q [51] Int. Cl. C22c 39/14 [58] Field of Search 75/126 A, 126 C, 126 E,75/126 H, 126 1,126 O [56] References Cited UNlTED STATES PATENTS2,241,187 5/1941 De Vries 75/126 0 [451 Nov. 26, 1974 1/1967 Steven75/126 Q 5/1974 Steven 75/126 A Primary ExaminerL. Dewayne RutledgeAssistant ExaminerArthur J. Steiner Attorney, Agent, or Firm-Cushman,Darby & Cushman [57] ABSTRACT 3 Claims, 3 Drawing Figures grind and alsoreduces their toughness.

HIGH-SPEED TOOL STEELS This invention relates to high-speed steelshaving a high wear resistance combined with good machinability andductility, and particularly relates to such steels wherein the siliconcontent is a function of the C/W/Mo/V content.

High-speed tool steels are characterized by high re- 5 I sistance towear up to working temperatures of 600C.

higher vanadium contents up to 5 percent, and higher correspondingpercentages of carbon. Although such steels are highly wear-resisting,their high content of coarse particles of vanadium carbide, which has ahardness of about 3000 Vickers, makes them difficult to Recently thecarbide content, and hence the wear resistance, has been raised byincreasing the carbon content. Examples embodying such practice are thesteels which in the A151 designation are known as M 41 and M 45, andwhich have vanadium contents of l to 2 percent with carbon contentsbetween 1.1 and 1.2 percent. However it has been found that an increasein carbon leads to the development of spiculate carbides of the type MC, instead of the usual high-speed steel carbide M C (DEW TechnicalReports 12, 1972, pp 111/33), and their hardness of around 2000 Vickersand unfavourable shape also adversely affect grindability and ductility.

The present invention is directed to solving the prob- 3 lem ofproducinghigh-speed tool steels which combine high wear resistance with goodgrindability and ductility.

The present invention is based on the surprising discovery that providedcertain alloying relationships are observed, namely in correlatingthesilicon content to Steel No.

. 2. pletely decomposes in a peritectoid reaction to the MC- and M Ccarbides which then appear in the desired form and distribution.

The invention provides a high-speed tool steel possessing high wearresistance combined with good ductility and machin'ability, consistingessentially of balance iron,

wherein the silicon content is not less than 0.5 percent and isdetermined by the formula Si 2.7 (%C) 0.1 (%W) 0.14 (%Mo) 0.5 @0113? 9-By the term consisting essentially of in the Specification and claimshereof, is meant that impurities andabove the quantity as hereinbeforeset forth leads to an undesirable carbide development and hence to animpairment of the properties of the steel. in such a case coarse primarycarbides of the type M C and MC appear.

l sman insla zlsa; H V

The content limits of the silicon that is to be introduced according tothe present'specification are calcu-.

' lated from the equation hereinbefore set forth by reference to thelimits in the content ranges of the melting y si g aa tsa nattisul rlywe!!! The compositions of the examined steels within the ranges embracedby the invention are listed in the ac- C Cr W Mo V C0 Si 1 1.05 4.5 1.39.1 1.05 7. balance 1.03 1.10 5.5 1.5 2.5. 1.25 8.4 1.35 2 0.98 4.0 6.2.0 1.75 .l. balance 0.56 1.03 4.5 6.5 5.25 2.0 0.59 3 0.98 4.0 1.5 8.01.75 V .l. balance 0.59 1.03 4.5 1.75 .5 2.0 0.94 4 0.97 5.0 2.5 2.752.25 J. balance 0.97 1.03 5.5 2.8 3.0 2.5 1.32

based on thg stated w limits of C. W. Mu. V l g to the w as herein setforth. W W

the contents of the alloying elements carbon. tungsten, molybdenum andvanadium, the carbide proportion in the structure can be increasedwithout the formation of carbides which by reason of their configurationand hardness impair grindability and ductility. This correlation betweenthe said alloying components according to the invention permits aperitectoid carbide reaction to take place which leads to fine andevenly distributed carbides and hence improves grindability andductility.

The addition of exactly predetermined quantities of silicon according tothe invention enables the carbon content to be raised and so adjustedthatalthough the carbide M C does form during solidification, it is sounstable that upon reheating prior to hot forming it comaccompanyingdrawings at a magnification x 500, of

which FIG. 1 shows that the steel containing 0.26 percent Si solidifieswith the formation of the spiculate M C carbide and that this remainsstable up to high degrees of deformation (FIGS. la c).

FIG. 2 shows that the Steel 1 having a silicon content according tothe'above formula also solidifies via the M C eutectic but that itscarbide is so unstable that this already decomposes as the ingot cools(FIG. 2a). This peritectoid decomposition becomes complete in the courseof the following hot working process. Very fine and evenly distributedcarbides of type M c and MC (FIGS. 2b and c) are formed, which impart tothe Steel 1 a high resistance to wear coupled with good grindability andtoughness.

FIG. 3 demonstrates that silicon contents exceeding that calculated bythe equation according to the invention lead to solidification of Steel1 via the M C eutectic, giving rise to the appearance of coarse primaryMC carbides (FIG. 30). These coarse carbides cannot be fully broken downeven by considerable deformation, and they lead to the development of anirregular coarse-grained structure which adversely affects theproperties of the said Steel 1.

The majority of conventional high-speed tool steels have siliconcontents that are too low for an optimal development of the carbides. Ifthe present invention is applied to such steels their properties areimproved, as illustrated by the following results.

With Steel 2, the plastic bending effort in a static bending test wasdetermined. The Steel 2 had been heat-treated as follows: Hardened byoil quenching from 1,200C; tempered at 560C for two periods of one hour.The measured value was 2.5 .I. By comparison the corresponding values ofsteels similar to Steel 2 but containing only 0.3 percent Si and 1.8percent Si, were about 2 J.

In order to test ductility and cutting performance planing tests werecarried outwith Steel 3. A tool made of the said Steel 3 was hardened byoil quenching from l,200C. and tempered at 550C. for two periods of onehour. The wear on the underface was measured after 1000 strokes(equivalent to 450 metres total length of cut). The wear in Steel 3 wasfound to be 0.1 mm. Compared with this result steels similar to Steel 3but containing 0.3 percent and 1.8 percent Si, showed twice as muchwear, viz. 0.2 mm.

In a turning test on blanks made of Steel 4, the durability, i.e., thecutting speed in m/min at which the tool failed, i.e.. became useless in30 minutes machining, was highest with a time of 19.5 m/min when thesilicon content had been adjusted to the contents of the other alloyingelements according to the invention. The tools made of Steel 4 werehardened by oil quenching from l,200C. and tempered at 540C. for twoperiods of one hour. However tests on steels of the composition similartoSteel 4, but in one instance containing 1.8 percent Si, and in anotherinstance 0.3 percent Si, gave V60 durability test results of l9 m/minand I8 m/min.

properties is obtained. Increased silicon contents lead to greaterhardness but they cause a shift of the secondary hardness maximum tolower annealing temperatures. This reduction in temper stability due tothe silicon is a disadvantage in the use of such steels. It has beenfound that this disadvantage may be overcome by means of furtheralloying procedures and that the reduction in temper stability due tothe silicon can be improved by raising the chromium content of thesteels which is normally about 4 percent to higher values.

Thus in a preferred embodiment of the invention the optimum chromiumcontent to ensure stability to temper should be related to the siliconcontent of the steel by the formula Cr=4 2 (%Si).

Silicon additions reduce the residual austenite which is present afterthe steels have been hardened. In order to mitigate this effect it istherefore advantageous to introduce into silicon-containing steelsaccording to the invention nitrogen up to 0.] percent.

The present invention enables high-speed tool steels of optimumstructure and hence of outstanding applicational properties, such astoughness, wear resistance and grindability to be produced. Despitetheir improved wear resistance steels composed as proposed by thepresent invention surprisingly still prove to be readily grindable.Whereas with grinding steels of corresponding other compositions thegrinding wheels quickly choked and became blunted causing inadmissibleheating and scorching of the ground tools, steels of the proposedcomposition could be ground substantially without any difficulty.

High-speed tool steels according to the invention are particularlysuitable for the making of tools which call for a considerable amount ofgrinding to give them their final shape, particularly twist drills,taps, cutters, reamers and milling tools.

What is claimed is:

I. A high-speed tool steel possessing high wear resistance combined withgood ductility and machinability, consisting essentially of 0.8 to 1.8'/1 carbon,

3.5 to l0 /1 chromium.

l to l3 tungsten,

l to 10 /1 molybdenum. 0.5 [0 5 71 vanadium. 0 to 10 "/1 cobalt.

balance iron.

wherein the silicon content is not less than 0.5 percent and isdetermined by the formula Si 2.7 (%C) 0.1 (%W) 0.14 (%Mo) 0.5 (%V) 0.3.

2. A high-speed tool steel according to claim 1, wherein the chromiumcontent is adjusted by reference to the silicon content according to theformula Cr=4+2 (%Si).

3. A high-speed tool steel according to claim 1,

wherein it contains up to 0.l percent nitrogen.

- l l =l

1. A HIGH-SPEED TOOL STEEL POSSESSING HIGH WEAR RESISTANCE COMBINED WITHGOOD DUCTILITY AND MACHINABILITY CONSISTING ESSENTIALLY OF 0.8 TO 1.8 %CARBON. 3.5 TO 10 % CHROMIUM, 1 TO 13 % TUNGSTEN, 1 TO 10 % MOLYBDENUM,0.5 TO 5 %VANADIUM, 0 TO 10 % COBALT, BALANCE IRON, WHEREIN THE SILCONCONTENT IS NOT LESS THAN 0.5 PERCENT AND IS DETERMINED BY THE FORMULA 2.A high-speed tool steel according to claim 1, wherein the chromiumcontent is adjusted by reference to the silicon content according to theformula % Cr 4 + 2 (%Si).
 3. A high-speed tool steel according to claim1, wherein it contains up to 0.1 percent nitrogen.